1. Field of the Invention
This invention relates to equipment for testing in situ the trip functions of circuit breakers which may have a range of rated currents and more than one trip function.
2. Background Information
Circuit breakers in some applications are tested periodically to assure reliability. This is particularly so in aircraft electrical systems. It is preferable that the circuit breakers be tested in their installed state. The job is made more difficult by the high density of these very small circuit breakers and the fact in any one aircraft system, the breakers can have a range of current ratings. Furthermore, aircraft circuit breakers are now being offered with multiple trip functions.
There is a need for an improved circuit breaker tester that is portable for use in situ testing of circuit breakers, and particularly, installations with circuit breakers having a range of current ratings and even multiple protection functions.
This need and others are satisfied by the invention which is directed to a tester for testing a plurality of circuit breakers each having a designated rated current within a range of rated currents. The tester includes a plurality of load elements each having a known fixed impedance. An electronic switch associated with each load element when turned on connects the associated load element in series with the load terminal of a circuit breaker under test and in parallel with any other load elements connected in series with the load element. A controller turns on a number of the electronic switches selected to collectively draw through the associated load elements a combined test current selected as a function of the designated load current for the circuit breaker under test and sufficient to trip the circuit breaker. The controller turns on the electronic switches in a pattern that draws current through the circuit breaker under test at an amplitude appropriate for the rated current of that circuit breaker and with a waveform appropriate for the protection function being tested. For example, for testing circuit breakers having a delayed trip function and an arc fault function, the controller turns on a first number of the electronic switches for the delayed trip function and a second number of the electronic switches for the arc fault protection function thereby providing separate reference values for the two different trip functions. For circuit breakers in ac electrical systems, the controller can turn on the appropriate number of electronic switches on just some half cycles to test the arc fault protection function, for instance, in the case where the circuit breaker detects arc faults by recognizing gaps in current due to the arcing. In a preferred embodiment of the invention, the controller includes a full wave rectifier so that the current through the load elements is dc. In this case, a zero crossing detector synchronizes turning on of the electronic switches with half cycles of the ac.
The controller includes an indicator means indicating the results of the test. Preferably, the controller includes a power supply connected to the line terminal of the circuit breaker under test so that the indicator is an electrically energized indicator which will operate even with the circuit breaker tripped. The controller also monitors the voltage on the load terminal and includes an arming means which is only activated when the voltage on the load terminal is not substantially zero, indicating that the circuit breaker to be tested is closed.
The controller includes means for indicating test failure if the circuit breaker under test does not trip within a selected time window, beginning a predetermined time after the test is initiated. This includes means indicating failure when the voltage on the load terminal of the circuit breaker under test goes to substantially zero before the selected time window opens or the voltage on the load terminal remains sufficient to indicate that the circuit breaker is still closed after the selected time window closes.
The controller also has a test mode for testing the tester. This includes means testing the impedance of the load elements. The self-testing means includes means checking that the voltage on the load terminal of the circuit breaker under test is not substantially zero, means sequentially turning on one electronic switch at a time, and means checking that the voltage across the associated load element remains within a selected range.